looking for source regarding special compressor surge detail 3

[passuff]

I'm writing my diploma thesis about some investigations on compressor surge. I was looking for an explanation of special surge detail regarding the surge frequency. In my experiments I always have the surge frequency of 20Hz multiple times (40Hz, 60 Hz) with weaker amplitude.
Today I finally found a paper mentioning this behavior.-->

http://www.gtisoft.com/confarch/BorgWarnerSurge2010.pdf

ON slide 12 of 18 you can see the frequency analysis of the pressure signal. The peaks at 14.6 and 21.9 Hz are mentioned to be described by Gravdahl (2004). I was searching for documents of Gravdahl from 2004 but I failed. I found his dissertation and many other papers regarding surge, but they did not explain the multiple frequencies...
I hope that someone on this board is able to help me, although this is a very special thematic...
regards
Michael from Germany

 

[MikeHalloran]

Are you asking why a periodic phenomenon has harmonics?

I'm pretty sure I don't understand your question.


Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Mike

So it's not just me.

I was thinking the frequency will depend on the geometry of the chamber the air is pumped into and excitation frequency from inlet valves opening and closing.

Regards
Pat
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[patprimmer]

Or the inertia in the wheels as the boost increases and stalls the turbo, then it spools down, then recovers and spools up again. Volumes in manifolds and inertia in the tubo set up thee oscillations.

Regards
Pat
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[MikeHalloran]

I once knew a guy who was going to make a fortune installing axial flow fans as grade retarders for trucks, based on his discovery that the fan power requirement goes up with the 5th (or some high Nth) power of rpm. I'm not sure he understood that such fans also stall. I guess he knows, now, because he's not Mister Grade Retarder.

The referenced slideshow links fan stall to Helmholtz resonance of the cavities downstream, which seems a credible explanation for surge. The slides appear to be a report/justification for a computer model of same. I wish there was a bit more detail on the real data used for correlation.



Mike Halloran
Pembroke Pines, FL, USA

 

[passuff]

OK. Maybe you're right and my text was a bit weird.
I understand why there is surge at 7.3Hz. This is caused by the resonance frequency of the system between compressor and engine.
It can be calculated with Helmholzresonator formula.
But as the simulation results show in the figure there are multiple of that also at 14.6Hz/ 21.9Hz.
I can reproduce this effect on my test bench, but I don't know what it is caused by. Why are there always multiple pump frequencies and not just one?
The answer to this question (as I understand) is linked to "Gravdahl 2004". But I can’t find any document from 2004 containing information about the multiple frequencies. I hope that somebody either know a source or maybe can explain this effect...

 

[MikeHalloran]

I suppose you could think of the Helmholtz resonance as a set of conditions that allow the existence of a standing pressure wave of frequency F. The same conditions also allow the existence of standing waves of frequency 2F, 4F, 8F, etc.

Harmonics are not limited to gas turbines; you can probably find a bucket of information in any text about vibrations.


Mike Halloran
Pembroke Pines, FL, USA

 

[SomptingGuy]

Take a look at the pressure signal in the time domain. If it isn't a pure sinusoid (unlikely from a system like that), you will always have harmonics of the fundamental frequency.

- Steve

 

[passuff]

Sorry for answering that late. First of all I would like to thank you for your replies.You pushed me into the rhight direction. I don't know much about oscillations. Now I know where to start reading.. (harmonics, standing wave, linear and non linear oscillations)
Thank you guys!